This invention relates to a high frequency power amplifier for use in a mobile radio communication terminal or the like required to operate with high efficiency.
In the field of mobile radio communication, systems utilizing the method of digital modulation are now increasingly put to practical use. In such a system, linear amplification is required in view of the method of modulation, and a power amplifier used in the system is not an exception.
FIG. 1 shows the structure of a conventional two-stage power amplifier commonly used in the field of mobile radio communication. The two-stage power amplifier shown in FIG. 1 consists of power amplifying FET's 1 and 2, and matching circuits 3, 4 and 5. FIG. 2 is a graph showing operating characteristics of such a power amplifier using a GaAs MESFET as its device. In FIG. 2 in which the horizontal axis represents an input to the power amplifier, and the vertical axis represents an output from the power amplifier, the curve A shows how the output changes relative to the input, the curve B shows current consumption of the power amplifier, and the curve C shows how adjacent channel interference power that is an important factor set forth in the Radio Transmitter Standards changes. The power amplifier is demanded to sufficiently satisfy the requisite of the adjacent channel interference power C set forth in the Standards when the power amplifier generates its required maximum output, and the design and selection of the device are further required so as to minimize the power consumption when the values of adjacent channel interference power are up to the Standards.
However, in the case of the conventional power amplifier described above, it is necessary to supply a sufficient amount of idling current in order to decrease modulation distortion that may occur when the power amplifier generates its required maximum output. This is because, as will be seen from the operation characteristics shown in FIG. 2, the current consumption B would not decrease even when the output A decreases, that is, in other words, the power conversion efficiency is very low.
In a digital cellular radio telephone system, transmission power changes over a wide range depending on the command from the base station or the received signal strength. Especially, in the case of the CDMA system, the transmitter output range is as large as 85 dB, and the effective power consumption during speech is substantially dependent on the current consumption at the time of power down. Therefore, how to reduce the current consumption by simple means at the time of power down is a great problem in the system of the kind described above.